Shutdown protecting relay is a component necessary to a mobile-protected electrical equipment, its main function is to ensure that the movable contacts and resting contacts of the mobile-protected electrical equipment, which carry electric current, can be engaged and disengaged quickly and reliably when power supply is on. Owing to the requirement of the power of electrical equipment, movable contacts and resting contacts of the mobile-protected electrical equipment must carry a larger current, so that the shutdown protecting relay must have the higher properties as follows:                1. when a mobile-protecting circuit is closed, that is, the movable contacts and the resting contacts are engaged with each other, a larger locking force must be kept between these two kinds of contacts;        2. when the mobile-protecting circuit is opened, that is, the movable contacts and the resting contacts are disengaged, a larger disengaging force is necessary to ensure the movable contacts and the resting contacts separated and to ensure a certain disengaging interval therebetween, meanwhile, the disengaging time must be very short;        
But above-mentioned two requirements are contradictory with each other.
For a mobile-protected electrical equipment, owing to limitation of its properties, the larger disengaging force and the quicker disengaging time must be emphasized first, therefore, for a sufficient locking force between the movable contacts and the resting contacts, a hidden peril has ever existed; in addition, most of components of shutdown protecting relay of the prior art are made of engineering plastics, the larger the current carried and the longer the power supply is on, the poorer the high-temperature performance of the shutdown protecting relay, which finally causes the shutdown protecting relay to be in failure, the equipment to be burnt-out, extremely results in the disastrous events such as arc fire and electroshock injury.